Stainless Steel Coil Tube (Coiltube) — Cold-Condition Heat Exchanger Series
1. Product Overview
The Stainless Steel Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd., is a precision-engineered continuous coil-form tubing product designed for high-efficiency heat exchange applications, particularly in low-temperature and cryogenic environments. Unlike straight-length tubes that require field fabrication and welding, coil tubes are delivered as continuous, pre-formed helical coils that eliminate the need for on-site cutting, bending, and joining — thereby significantly reducing installation labor, minimizing potential leak points, and improving overall system reliability.
This product category is classified under Literature Study (文献学习) and is informed by in-depth technical research into the performance characteristics and failure modes of coil-type heat exchangers under cold conditions, including frost formation dynamics, thermal contraction stresses, and low-temperature brittleness concerns. The design philosophy integrates lessons learned from field failures in cryogenic service to deliver a product that maintains mechanical integrity and thermal performance across an extended operating temperature range from −196 °C (Liquid Nitrogen temperature) to +600 °C.
Unique Selling Point: The coil tube geometry inherently provides superior heat transfer coefficients compared to straight-tube configurations due to the enhanced turbulence induced by the helical curvature. Studies have demonstrated that the Nusselt number (Nu) in coiled tubes can be 20–40% higher than in equivalent straight tubes at the same Reynolds number, owing to secondary Dean vortex flow patterns that continuously refresh the boundary layer.
2. Material & Properties
2.1 Base Materials
Coil tubes are fabricated from a range of austenitic and duplex stainless steel grades, selected according to the specific service environment. The most commonly offered grades include:
- Austenitic Grades: AISI 304 (UNS S30408), AISI 316 (UNS S31608), AISI 316L (UNS S31603), AISI 321 (UNS S32108), AISI 347 (UNS S34708)
- Super-Austenitic Grades: Alloy 254 SMO (UNS S31540), Alloy 625 (UNS N06625)
- Duplex Grades: 2205 (UNS S32205), 2507 Super Duplex (UNS S32750)
- Cryogenic-Grade Materials: AISI 304L, AISI 316L (preferred for temperatures below −40 °C due to lower carbon content ensuring excellent low-temperature ductility)
2.2 Mechanical Properties (Typical — AISI 316L Annealed Condition)
- Tensile Strength (UTS): ≥ 485 MPa (70 ksi)
- Yield Strength (0.2% Proof): ≥ 205 MPa (30 ksi)
- Elongation at Break: ≥ 40% (in the transverse direction)
- Hardness: ≤ 230 HB (Brinell)
- Impact Energy at −196 °C: ≥ 100 J (Charpy V-Notch, transverse)
- Thermal Conductivity (20 °C): 16.3 W/(m·K)
- Linear Expansion Coefficient (20–100 °C): 17.3 × 10⁻⁶ /°C
- Electrical Resistivity (20 °C): 0.74 μΩ·m
2.3 Corrosion Resistance
The austenitic stainless steel composition provides excellent resistance to general corrosion, pitting, and crevice corrosion in a wide variety of media. The addition of 2.0–3.0% Molybdenum (Mo) in 316L grade significantly enhances pitting resistance, as quantified by a Pitting Resistance Equivalent Number (PREN) of approximately 24–26. For severe chloride-containing environments (e.g., seawater, chemical processing), Alloy 254 SMO (PREN ≥ 44) or Alloy 625 are recommended.
2.4 Cold-Condition Performance Characteristics
A critical differentiator of this coil tube product is its proven performance under cryogenic and sub-zero conditions. Austenitic stainless steels exhibit a unique metallurgical advantage: unlike ferritic and martensitic steels, they do not undergo a ductile-to-brittle transition at low temperatures. The face-centered cubic (FCC) crystal structure of austenite remains stable down to absolute zero, ensuring that the material retains full ductility and toughness even at liquid nitrogen temperatures (−196 °C).
Furthermore, the coil geometry itself offers thermal advantages in cold service:
- Reduced thermal bridging compared to straight-tube bundles, limiting cold spots on the shell wall
- Lower installed mass per unit heat transfer area, reducing the thermal contraction stress magnitude
- Continuous coil construction eliminates field welds — a primary failure mode in cryogenic heat exchangers subject to thermal cycling
3. Specifications
| Parameter | Specification Range | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (¼") to 114.3 mm (4½") | ASTM A213 / ASME SA-213 |
| Wall Thickness | 0.5 mm (0.020") to 6.35 mm (¼") | ASTM A213 / ASME SA-213 | Coil Diameter (Helical Mean Diameter) | 100 mm (4") to 2,000 mm (80") | Custom per project |
| Coil Pitch (Helix Pitch) | 20 mm to 200 mm | Custom per project |
| Coil Length (Continuous Length) | 10 m to 500 m (continuous, unspliced) | — |
| Surface Finish | Pickled & Passivated (P&P), Ra ≤ 1.6 μm; optional electropolished Ra ≤ 0.4 μm | ASTM A967 |
| End Preparation | Plain, beveled, threaded (NPT/BSP), or flanged (ASME B16.5) | ASME B16.5 / ASME B1.20.1 |
| Operating Temperature Range | −196 °C to +600 °C | — |
| Maximum Operating Pressure | Up to 40 MPa (5,800 psi), dependent on wall thickness and coil diameter | ASME BPV Code Section VIII |
| Applicable Media | Water, steam, hydrocarbons, ammonia, refrigerants, chemicals, food-grade fluids | — |
| Standards Compliance | ASTM A213, ASME SA-213, EN 10216-5, GB/T 12771, JIS G3459 | — |
| Heat Treatment | Solution Annealed (1050–1150 °C, water or air quenched), followed by pickling and passivation | ASTM A967 |
| Maximum Heat Treatment Temperature | 1150 °C (2,100 °F) | — |
3.1 Typical Grade Selection Guide
| Grade | PREN | Cryogenic Suitability | Recommended Application |
|---|---|---|---|
| 304 / 304L | 19 | Excellent (down to −196 °C) | General chemical, food processing, cryogenic |
| 316 / 316L | 25 | Excellent (down to −196 °C) | Marine, chlorinated environments, cryogenic |
| 321 / 347 | 19 | Good (down to −100 °C) | High-temperature service, thermal cycling |
| 2205 Duplex | 35 | Limited (down to −60 °C) | High-pressure, chloride-containing |
| Alloy 625 | 38 | Excellent (down to −269 °C) | Severe corrosion + cryogenic |
| 254 SMO | 44 | Excellent (down to −196 °C) | Extreme chloride, pharmaceutical |
4. Manufacturing Process
The production of high-quality stainless steel coil tubes follows a rigorous, multi-stage manufacturing workflow designed to ensure dimensional accuracy, metallurgical soundness, and surface integrity:
- Raw Material Procurement & Inspection: Cold-rolled stainless steel strip or seamless pipe stock is sourced from certified mills. Each batch is accompanied by a mill test certificate (MTC) conforming to EN 10204 3.1. Incoming inspection includes visual examination, ultrasonic testing (UT) per ASTM E165, and chemical composition verification via Optical Emission Spectrometry (OES) or X-Ray Fluorescence (XRF).
- Coil Forming (Helical Bending): The tube is helically formed using a CNC-controlled coil winding machine. The forming process is performed at controlled speeds to prevent work hardening, plastic deformation, or ovality. The resulting coil must meet strict geometric tolerances:
- Coil diameter tolerance: ±0.5%
- Pitch uniformity: ±1.0 mm
- Ovality (roundness deviation): ≤ 1.0% of nominal OD
- Wavelength deviation: ≤ 0.5%
- Solution Annealing: Following coil forming, the tubes are solution annealed at 1050–1150 °C to relieve residual stresses, restore full solutionized microstructure, and ensure maximum corrosion resistance. Annealing is performed in a continuous belt furnace with a controlled reducing or inert atmosphere (dew point ≤ −40 °C) to prevent scale formation and intergranular chromium carbide precipitation.
- Pickling & Passivation: The annealed tubes undergo acid pickling (typically a mixture of nitric and hydrofluoric acid) to remove any residual scale or oxide. This is followed by passivation with a nitric acid or citric acid solution to form a stable, protective chromium oxide passive film. Post-passivation surface cleanliness is verified by the Rust-Test (ASTM A967 Method C) and the Blue-Prussian-Test (ASTM A967 Method D). Surface roughness is measured via contact profilometry, targeting Ra ≤ 1.6 μm (or Ra ≤ 0.4 μm for electropolished finish).
- Hydrostatic Testing: Each coil tube assembly is subjected to 100% hydrostatic pressure testing at 1.5 times the maximum design pressure, with a minimum hold time of 60 seconds. Testing is performed per ASTM A213 / ASME SA-213 requirements. Any leakage indicates rejection.
- Dimensional Verification & Final Inspection: Comprehensive dimensional checks include OD, wall thickness (measured at multiple points along the coil using ultrasonic wall thickness gauges per ASTM E797), coil diameter, pitch, and overall length. Surface quality is inspected visually and, where required, by borescopic examination.
- Packaging & Shipping: Coils are wrapped in moisture-barrier protective film, placed on wooden or steel saddles, and secured to prevent deformation during transport. Each shipment includes a detailed packing list, material traceability documentation, and test certificates.
4.1 Process Quality Control Milestones
| Stage | Inspection Method | Acceptance Criteria |
|---|---|---|
| Incoming Material | UT (ASTM E165), OES/XRF | No internal defects; composition within ASTM specification limits |
| Post-Forming | Visual + dimensional measurement | Ovality ≤ 1.0%; no surface cracks or folding |
| Post-Annealing | Microstructural examination (optical metallography) | ≥ 95% recrystallized austenite; no carbide precipitation at grain boundaries |
| Post-Pickling/Passivation | Rust-Test (ASTM A967 C), surface roughness profilometry | No rust spots after 24-hour exposure; Ra ≤ 1.6 μm |
| Final | Hydrostatic test, UT wall thickness, dimensional audit | No leakage; wall thickness ≥ specified minimum; dimensions within tolerance |
5. Applications
The stainless steel coil tube product is widely deployed across multiple industrial sectors, with particular emphasis on applications involving heat exchange under cold or cryogenic conditions:
5.1 Cryogenic & Low-Temperature Systems
- Liquefied Natural Gas (LNG) Facilities: Coil tubes used in LNG pre-coolers, methane/propane heat exchangers, and cold box internals operating at temperatures as low as −162 °C. The continuous coil construction eliminates the need for thousands of field welds, which are a critical failure risk in cryogenic service.
- Liquid Nitrogen / Liquid Oxygen Systems: Cryogenic storage tank cooling coils, LN₂ transfer lines, and oxygen plant heat recovery exchangers.
- Cryogenic Distillation: Air separation units (ASUs) where coil-type heat exchangers recover cold energy between process streams.